This invention relates generally to the controlling of pen-to-print medium spacing on a wet ink printer. More particularly, the invention concerns an apparatus for adjusting the pen-to-print medium spacing automatically according to a selected thickness of a print medium.
Typically an ink-jet printer, or any printer using wet ink, include a pen (also called a printhead) and a print platen for supporting a print medium for printing with the pen. An area between the pen and the print platen is commonly known as a print zone of the printer. The print platen guides and supports the print medium in the print zone during printing. The printer also includes a print medium feed mechanism for feeding a print medium through the print zone. During printing, ink is placed on the print medium by dropping or ejecting the ink from the pen, or by any other printing method well-known by those skilled in the art. The quality of a printout on the print medium depends on the resolution of the printer. The resolution is defined as the number of drops of ink required to cover a given area. For example, a printer with a 600 dots-per-inch (dpi) resolution is able to print dots of a size of {fraction (1/600)} of an inch. To achieve higher resolution and thus higher quality printing, it is a constant goal to achieve even smaller dot sizes from the pen. In addition to dot size, it is crucial that the drop be placed accurately on a desired position on the print medium. Inaccuracy in placement will result in a printout that lacks sharpness. Also inaccurate placement of dots will affect the colors of a printout since the colors are obtained by a half-toning process. There are several factors affecting the accuracy of placement of ink drops. These factors include the control of the movement of the pen, the timing of firing pulses applied to the pen and other known factors. One factor affecting placement accuracy is the draft that is created by movement of the pen during printing. To reduce the effect of the draft, a print medium is brought as close to the pen as possible. The distance between the pen and the print platen supporting the print medium is known as the pen-to-print medium spacing or distance. The smaller the pen-to-print medium distance, the less likely printing is affected by the draft. However, there is a limit to this reduction of pen-to-print medium distance. When a print medium is brought too close to the pen, smearing will occur during printing. Ink used in wet ink type printing includes a relatively large amount of water. As the wet ink comes into contact with the print medium, the water in the ink saturate the fibers of the print medium, causing the fibers to expand, which in turn causes the print medium to buckle. Such buckling will cause the print medium to come into contact with the pen during printing. Therefore, some allowance is required to prevent such a buckling print medium from touching the pen. Typically in the production of such printers, the pen-to-print medium distance is calibrated for a commonly used media thickness of for example 0.1 mm. With this media thickness, the printer mechanism is adjusted such that a good quality printout is achieveable. Because of the requirement to support media of different thicknesses, some printers are provided with mechanical levers for a user to manually adjust the pen-to-print medium spacing. Usually two values of pen-to-print medium spacing are provided, one for thinner media and the other for thicker media. One disadvantage of such a system is that the quality of printing is contingent on the user remembering to move the lever to the correct position for a print medium. If the pen-to-print medium spacing is incorrectly set, poor quality printout will result. For example if high pen-to-print medium distance is selected for a thin medium, the earlier mentioned problem of draft will affect the accuracy of the placement of the ink drops. If low pen-to-print medium is selected for a thick medium, smearing may occur. It is therefore important that a user sets the lever to the correct position before commencing printing.
To overcome this problem of a user having to properly set the pen-to-print medium distance, some printers are designed to detect the widths of a print medium and to adjust the pen-to-print medium spacing accordingly. However, such a design is restrictive in the sense that it accepts only certain print media of the appropriate size and thickness.
From the foregoing, the prior art therefore has a need for an improved method and apparatus for adjusting pen-to-print medium spacing which is less error prone and which is able to accept media of different sizes and thicknesses.
In accordance with a preferred embodiment of the present invention, an apparatus for adjusting pen-to-print medium spacing in a printer has a pen and a print platen. The print platen supports a print medium for printing using the pen. The apparatus also includes a datum for holding the print platen a first predetermined pen-to-print medium spacing away from the pen. The print platen is resiliently biased against the datum. The print platen can be moved away from the datum to define a gap therebetween. The apparatus further includes an arm which is moveable into and out of the gap. When the arm is in the gap, the print platen rests against the arm to define a second predetermined pen-to-print medium spacing. The arm is preferably moved by a means which is activated independently of the size of a print medium.
Preferably, this arm moving means includes a first member which has a first cam fixed to a second cam. The first cam has a contour on which the arm rides for the arm to be moved into and out of the gap. The second cam has a contour for tilting the print platen away from the platen when the arm is moved. The first member further includes a main gear fixed to either the first or the second cam. This main gear derives its movement from a motor to rotate the cams. The first member also has a locating means for allowing the position of the cams to be determined.
According to the preferred embodiment, this locating means includes two substantially diagonally disposed toothless sections of different lengths on the main gear. To determine the position of the cams, the arm-moving means further includes a rocker arm. This rocker arm has a first portion and a second portion. The first portion and the second section are angularly disposed to each other. Slidably coupled to the rocker arm is a pivot gear. This pivot gear and the rocker arm are pivotably mounted so that the rocker arm tilts when the pivot gear is rotated. Pivotably mounted on the first portion and second portion are a first gear and a second gear respectively. These first and second gears derive their movement from the pivot gear to drive the main gear in a predetermined direction. At any one time, only one of the first and the second gears is allowed to engage the main gear. When one of the toothless sections is rotated to be adjacent to the first gear, the second gear is adjacent to a toothed section of the main gear. With such an arrangement, a longer of the two toothless sections is driven adjacent to the first gear to define an initialized position of the cams. This position of the cam corresponds to one of the pen-to-print medium spacings. From this known position, the cams can be rotated to alternate between positions corresponding to the first and the second pen-to-print medium spacings.
The rocker arm has a latching means for activating and deactivating the apparatus. During a printing mode, the rocker arm is latched to deactivate it. During an adjustment mode, the latching means releases the rocker arm to allow the rocker arm to be used for adjusting the pen-to-print medium spacing. Such an apparatus allows its movement to be derived from a motor which is used in the printer for advancing a print medium during printing.
The apparatus also allows the pen-to-print medium spacing to be adjusted independently of the size of a print medium. In order for the printer to carry out an adjustment, a parameter indicating the desired pen-to-print medium spacing is remotely sent to the printer.